Comparative Transcriptome Profiling Indicated that Leaf Mesophyll and Leaf Vasculature have Different Drought Response Mechanisms in Cassava

Drought stress is one of the major environmental factors that limited crop’s growth and production. Cassava known as a tropical crop that is widely distributed in Sub-Saharan Africa. It has a strong drought tolerance and can grow well under tough environmental conditions. Therefore, understanding how cassava responds to drought stress and coordinates survival and accumulation has great theoretical significance for improving crop drought resistance breeding. Many studies on cassava drought responses mainly focused on the leaf and whole seedling. Nevertheless, how the vasculature plays an important role in plant response to water deficiency remains to be fully elucidated. Here, comparative transcriptome analysis was performed on isolated mesophyll tissue and leaf vein vascular tissue of cassava variety KU50 after mild drought treatment to determine the molecular mechanism behind drought resistance in cassava vasculature. Our results showed that KU50 leaves had increased leaf temperature, with characters of rapidly decreased net photosynthetic rate, stomatal conductance, and transpiration rate in leaves, and the intercellular CO2 concentration accumulated under drought stress. Comparative transcriptome profiling revealed that under drought stress, leaf mesophyll tissue mainly stimulated the biosynthesis of amino acids, glutamic acid metabolism, and starch and sucrose metabolism. In particular, the arginine biosynthesis pathway was significantly enhanced to adapt to the water deficiency in leaf mesophyll tissue. However, in vascular tissue, the response to drought mainly involved ion transmembrane transport, hormone signal transduction, and depolymerization of proteasome. Concretely, ABA signaling and proteasome metabolism, which are involved in ubiquitin regulation, were changed under drought stress in KU50 leaf vascular tissue. Our work highlights that the leaf vasculature and mesophyll in cassava have completely different drought response mechanisms.

Genome-Wide Identification and Expression Analysis of AP2/EREBP Transcription Factors in Litchi (Litchi chinensis Sonn.)

APETALA2/ethylene response element binding proteins (AP2/EREBP) are a vital type of TF involved in plant organ development and embryogenesis. In this study we identified 202 Litchi AP2/EREBP TFs from the litchi genome. They were classified into four subfamilies by phylogenetic clustering, including AP2s (20), ERFs (112), DREBs (64), and RAVs (6). Analysis of conserved domains, motifs, gene structure, and genome localization were carried out to investigate the evolutionary features of litchi AP2/EREBPs. Over 35% of DREBs and ERFs involved in the expansion of litchi AP2/EREBPs resulted from tandem duplication. The majority of genomic organizations were conservative, except those of the AP2 subfamily, which had no intron and contained less conservative motif numbers. The expression profiles of litchi AP2/EREBPs in ten tissues were investigated using RNA-Seq data and fifty-nine showed tissue-specific expressions. Their expression patterns were confirmed by qRT-PCR with eight tissues-specificity genes. Six genes related to embryogenesis were identified using the map of orthologous gene interaction between Arabidopsis and litchi. This paper is a comprehensive report on the characteristics of the litchi AP2/EREBP gene superfamily. It will serve to further explore the regulatory mechanisms of AP2/EREBP TFs in the litchi somatic embryogenesis and provide information for litchi molecular breeding.